There is no sauce quite like béarnaise sauce, a hot emulsification that will make any food, and especially fresh white asparagus, or french fries in place of ketchup.
Bunch fresh chervil/tarragon/parsley
1 bay leaf
White wine vinegar
1 tsp crushed peppercorns
You use a bain marie to prepare the béarnaise sauce or a double boiler: http://en.wikipedia.org/wiki/Bain-marie
Finely cut the shallots and add the wine and vinegar, bay leaf and herbs. Add the herbs with parsley stalks and fresh peppercorns – reduce.
Filter out the herbs and discard them retaining the liquid.
Pour the reduction into the bain marie to make sure whatever it is not placed directly over a flame or it could break and the béarnaise spoils.
Let the vinegar cool a bit, then add the egg yolks. If the vinegar’s temperature exceeds 70°C, it might coagulate and spoil.
Now add; tablespoon by tablespoon the butter is added at room temperature, then whisk it into the egg, a water vinegar mixture.
As soon as one tablespoon of butter has been fully absorbed in the sauce continue with another tablespoon of butter. Continue until you run out of butter and if you need feel the consistency requires adding some water during the emulsification process, add it.
Finish by adding some freshly snipped herbs into the sauce, or if you prefer.
There are variations when it comes to making béarnaise sauce, one chef named Alexandre Étienne Choron had added tomato purée near the end. The other variation is named glace de viande, a concentrated reduction of brown stock, added to the basic Béarnaise.